The present invention relates to a device to be included in a printing machine for retaining a master stencil sheet, and more particularly relates to a device for attaching and retaining the leading edge of a stencil master sheet along a generator of a printing drum incorporated in a printing machine.
Various currently known types of printing machine incorporate a printing drum device around which a stencil master sheet is wrapped for performing the printing process; typically, in the use of such a printing machine, ink for printing is supplied from the inside of the printing drum via a large number of small holes in its outer surface to the inner side of the stencil master sheet wrapped therearound, and passes (only) through the perforated portions of said stencil master sheet to the outer side of said sheet, so as to be imprinted on a printing sheet such as a paper sheet and so as to duplicate the pattern perforated on said stencil master sheet. In such a printing machine, a device is required for securing one edge, typically the leading edge, of the stencil master sheet to the printing drum, typically along one of the generators thereof, in order securely to fix said stencil master sheet to the printing drum during the stencil printing process; and this securing or retaining device is required selectively to be actuatable to thus fix the edge of the master sheet to the printing drum, or to be actuatable to release said edge of said master sheet. Various such stencil master retaining devices have been proposed, and in particular the assignee company of the present patent application has proposed, in Japanese Patent Applications Ser. Nos. Sho 57-21845 (1982) and Sho 57-207217 (1982), a clamp type stencil master sheet retaining device, in which a magnet plate is fitted generally along a generator of the printing drum, and a movable clamp plate is selectively either held away from said magnet plate or is allowed to be attracted against said magnet plate by the magnetic force thereof. Thus, when it is desired to retain the leading edge of a stencil master sheet along said generator of said printing drum, then said leading edge is placed over the magnet plate, and the clamp plate is allowed to be attracted against said magnet plate while pinching said leading edge therebetween and thus retaining said stencil master sheet; but, when it is desired to release said stencil master sheet, then said clamp plate is held away from said magnet plate, thus letting go of said leading edge of said stencil master sheet.
This type of stencil master retaining device is very adequate for use in the case that the leading edge of the stencil master is thickened and reinforced by a retainer reinforcement strip; however, in the case that the stencil master sheet is thin and is of the so called headless type in which its leading edge is not particularly differentiated from its main body and is not thickened or reinforced (and this is a type particularly suited to automatic operation, since such stencil master sheets can be cut successively from a long roll of stencil master), then the difficulty can arise that, when the clamp plate is raised away from the magnet plate to release the leading edge of said stencil master, said leading edge does not properly become separated from said magnet plate against which it was clamped, but remains attached thereto (and thereby to said printing drum) by the effect of static electricity or the like. In this event, the subsequent operation of removal of the stencil master sheet from the printing drum becomes difficult and may not be successful. Particularly when the removal and disposal of the stencil master sheet is being performed by an automatic device, such for example as those disclosed in Japanese Utility Model Applications Serial Nos. Sho 54-17959 (1979), Sho 56-97030 (1981), or Sho 57-63378 (1982), then there is a great danger of the occurrence of an ejection failure with the printing master not being properly peeled away from the printing drum. In particular, a printing master sheet such as the type used in the thermal perforation method which is extremely thin and has a very smooth surface will particularly tend to adhere to the surface of the magnet plate, and will be particularly prone to ejection failure.